This invention relates to an optical waveguide with a multi-layer core and in particular to an optical waveguide with a composite core in which the consolidation temperature of a first core layer is above the softening temperature of a second core layer deposited thereon.
Planar waveguides are fabricated by forming several layers on top of a substrate, usually a silicon wafer. In the case of a FHD fabrication process, the layers which make up the waveguide are first deposited as a layer of fine glass particles or xe2x80x9csootxe2x80x9d. Alternatively the glass can be deposited by a variety of other techniques, for example, plasma enhanced chemical vapour deposition (PECVD), low pressure chemical vapour deposition (LPCVD), which may be done in isolation or combination and further may be in combination with flame hydrolysis deposition (FHD).
In the case of the FHD process the soot layers are consolidated into denser glass layers, either individually immediately after each layer is deposited or several layers may be consolidated together. In the case of the other processes, although deposited as a glass a densification and/or desiccation procedure is often also employed. If a layer is heated to a sufficiently high temperature in excess of its consolidation temperature, the viscosity of the consolidated layer is reduced until eventually the glass is able to flow. When this occurs, surface irregularities can be removed as the surface of the layer is smoothed.
During fabrication of an optical planar waveguide, it is know to consolidate a core layer using a temperature cycle in which at one stage the layer is heated to the xe2x80x9csofteningxe2x80x9d temperature, which is significantly higher than the actual consolidation temperature. This enhanced temperature stage ensures that the glass forming the core layer is sufficiently softened at its top surface for the consolidated core layer to flow and form a relatively smooth and level layer.
The smoother the surface of a waveguide the less light is scattered at the surface; heating a layer to its softening temperature for a period of time is therefore desirable if a high-quality waveguide is to be fabricated. However, to ensure that the underlying layers are not deformed during the consolidation and/or softening of subsequent layers, the consolidation and softening temperatures of each subsequent layer are usually less than the softening temperature of the underlying layer.
In order to achieve a suitably smooth core layer upper surface, without reaching temperatures which exceed the consolidation temperatures of the underlying layers and/or which could cause thermal deformation of the waveguide""s substrate, it is usually desirable to introduce selected dopants into the core layer during the deposition stage.
In the present invention the composition of the glass forming the lowest core layer is thus selected so that its refractive index is close to that of the overlying core layer(s) whilst its consolidation temperature is greater than the softening temperature of the topmost overlying core layer. Similarly, the cladding formed around the core layers and under the core layers must have the correct thermal characteristics to ensure that the core is not deformed during fabrication of the waveguide.
As a consequence all layers (buffer if employed, core and cladding) must be deposited with decreasing consolidation temperature and sufficient buffer in between each. In addition, the maximum consolidation temperature allowed, typically for the core layer, is limited to xcx9c1360xc2x0 C. by the onset of striations and implosions due to the silicon substrate.
The selected core dopants lower the temperature at which the top surface of the core layer begins to f low. For example, dopants such as boron, phosphorous and/or titanium ion species may be introduced into germano silicate glass during the deposition stage in selected quantities to give the desired properties, for example; the right thermal characteristics, refractive index and coefficient of expansion. Other co-dopants could include tantalum, aluminium, lanthanum, niobium and zirconium. Germano silica based core glass is the preferred example but germania may not be necessary in all cases.
The invention seeks to provide several advantages in the fabrication of an optical waveguide. The waveguide according to the invention has a composite core in which a first layer comprises a glass whose consolidation temperature is close to the maximum allowed (xcx9c13600xc2x0 C.). A xe2x80x9cskinningxe2x80x9d layer is then deposited on top of the underlying core layer(s) whose thickness is only of the order of ten percent of the thickness of the underlying core layer(s). Generally, the xe2x80x9cskinningxe2x80x9d layer has a much increased dopant concentration but match the refractive index of the underlying core layer(s). This uppermost xe2x80x9cskinningxe2x80x9d layer typically has a consolidation temperature xcx9c50xc2x0 C. less than the consolidation temperature(s) of the underlying core layer(s). The uppermost xe2x80x9cskinningxe2x80x9d layer fully consolidates and, due to its softening temperature being lower than the consolidation temperature(s) of the underlying core layer(s), is further softened. This promotes a surface xe2x80x9cskinning effectxe2x80x9d which gives rise to a low surface roughness. The region of increased dopant is thus minimised, and is located, for example, at the edge of the waveguide core where the optical field of the guided mode is minimised: the impact of any density fluctuations is thus reduced.
In order to ensure that both the consolidation and the softening temperatures of the core layer are sufficiently low, the core layer needs to be quite heavily doped. At such high levels of concentration, the dopants are more susceptible to non-uniform distribution within the core layer, and this results in the core layer exhibiting an undesirably high level of density fluctuations. The presence of density fluctuations affects the consistency of the refractive index across the layer, which should be as uniform as possible if the waveguide is to be used in large scale applications, for example, such as an array waveguide grating. The minimisation of such density fluctuations is particularly desirable in the fabrication of large-scale waveguides, for example, waveguides whose dimensions are in excess of 2xc3x972 xcexcm2.
Furthermore, when cladding the core, since the volume of the softer core glass is minimised a closer match in consolidation temperature between the clad and core layers can be employed before significant deformation of the core layer is observed.
During the consolidation phase, there is a reduction in surface area whilst at the same time an increase in density of the deposited layer. Necking between the deposited soot particles forms an open network with pores, which subsequently densifies with closure of the pores. Thus, it is essential that the consolidation conditions employed ensure that the lower viscosity uppermost (or xe2x80x9cskinningxe2x80x9d layer) does not consolidate prematurely.
Poor consolidation conditions may give rise to gas trapping problems which would damage the consolidating layer(s). To mitigate this, a suitable consolidation ramp temperature rate, such as for example 5xc2x0 C./min, may be used which enables the consolidating layer to be formed bubble free. He gas can also be used as it aids sintering by promoting core collapse.
The present invention seeks to obviate or mitigate the aforementioned disadvantages by providing a waveguide with a multi-layer core which has a uniform refractive index and a smooth uppermost surface.
A first aspect of the invention seeks to provide an optical waveguide with a multi-layer waveguide core, the waveguide comprising:
a substrate;
a waveguide core formed on the substrate; and
at least one upper cladding layer embedding said waveguide core, the waveguide core having a composite core layer comprising:
a first core layer with a softening temperature T1S formed on the substrate; and
at least one other core layer formed on the first core layer, wherein the softening temperature T2S of at least one of said at least one other core layers is less than the softening temperature T1S of an underlying core layer.
Preferably, the softening temperature T2S of at least one of said at least one other core layers is at least 10xc2x0 C. less than the softening temperature T1S of a least one underlying core layer.
Preferably, the softening temperature T2S of at least one of said at least one other core layers is substantially equal to or less than a consolidation temperature T1C of at least one underlying core layer.
Preferably, said substrate is silicon.
Preferably, said substrate further comprises at least one buffer layer formed thereon.
Preferably, at least one said buffer layer is a thermally oxidised layer of the substrate.
Preferably, at least one layer of said: first core layer, said at least one other core layer, and/or said at least one upper cladding layer comprises silica and/or germanium oxide.
More preferably, at least one of said first core layer, said at least one other core layer, and/or said at least one upper cladding layer is doped with at least one ion species taken from the group consisting of:
phosphorus, boron, titanium, tantalum, aluminium, lanthanum, niobium, zirconium and/or any other transition element.
More preferably, said at least one silica and/or germanium oxide layer is doped with at least one species taken from the group consisting of:
a transition element, a rare earth ion species and/or a heavy metal ion species.
Preferably, the thickness of first core layer is greater than the thickness of said at least one other core layer.
More preferably, the thickness of the first core layer is the major portion of the thickness of the composite core layer.
According to a second aspect of the invention, there is provided a method of fabricating an optical waveguide with a waveguide core comprising the steps of:
forming a substrate;
forming a composite core layer on said substrate;
forming a waveguide core from said composite core layer; and
forming at least one upper-cladding layer to embed said core waveguide, wherein the formation of the composite core layer is characterised by:
forming a first core layer with a softening temperature T1S on the substrate; and
forming at least one other core layer on the first core layer, wherein the softening temperature T2S of at least one of said at least one other core layer is substantially less than the softening temperature T1S of at least one underlying core layer.
Preferably, the softening temperature T2S of at least one of said at least one other core layer is at least 10xc2x0 C. less than the softening temperature T1S of at least one underlying core layer.
More preferably, the softening temperature T2S of at least one of said at least one other core layer is substantially equal to or less than a consolidation temperature T1C of at least one underlying core layer.
Preferably, at the softening temperature T2S of the said at least one other core layer, the viscosity of the said at least one other core layer is sufficiently reduced to lessen surface irregularities in said at least one other core layer.
Preferably, said step of forming a substrate includes the formation of at least one buffer layer on said substrate.
Preferably, the formation of at least one of: said at least buffer layer, said first core layer, said at least one other core layer, and said upper cladding layer comprises the steps of:
depositing a soot layer of fine particulate material;
consolidating said deposited soot layer.
Preferably, said soot deposition is by a flame hydrolysis deposition process, and/or any other planar soot deposition technique or combination of soot depositing techniques and non-soot depositing techniques.
More preferably, said consolidation is by heating with a flame hydrolysis burner and/or in a furnace.
Preferably, the formation of at least one of: said at least buffer layer, said first core layer, said at least one other core layer, and said upper cladding layer comprises the steps of:
depositing said layers of material by means of a plasma enhanced chemical vapour deposition process, a low pressure chemical vapour deposition process and/or any other planar deposition technique or combination of deposition techniques;
subjecting the deposited layer to a temperature controlled environment such that said deposited layer is sintered.
Preferably, the composition of at least one layer of said: first core layer, at least one other core layer, and/or said at least one upper cladding layer includes silica and/or germanium oxide.
More preferably, at least one layer of said: first core layer, said at least one other core layer, and/or said at least one upper cladding layer is doped with at least one ion species taken from the group consisting of:
phosphorus, boron, titanium, tantalum, aluminium, lanthanum, niobium, zirconium and/or any other transition element.
Preferably, at least one silica and/or germanium oxide layer is doped at least one ion species taken from the group consisting of:
a transition element, a rare earth ion species and/or a heavy metal ion species.
Preferably, the quantities of dopant are selected to form a waveguide with a refractive index difference of between 0.2-2% with respect to the buffer.
The lower core layer may be SiO2 co-doped with a Germanium and/or Boron and/or Phosphorus ion species. The upper core may be SiO2 co-doped with a Germanium and/or a Boron and/or a Phosphorus ion species.
Preferably, the softening temperature (T2S) of the said at least one other core layer is at least 10xc2x0 C. less than the consolidating temperature (T1C) of said first core layer.
The consolidation temperature (T1C) of said first core layer may be in the range 1200xc2x0 C.-1375xc2x0 C.
Preferably, the consolidation temperature T2C of the second core layer is between 1100xc2x0 C. to 1365xc2x0 C.
Preferably, the composition and concentration of dopants in any one lower layer and/or substrate is selected to control the degree of softness exhibited by any overlying layer at a predetermined temperature.
Preferably, during the consolidation, the temperature conditions include a stage where the temperature gradient rises at 15xc2x0 C. minxe2x88x921 between 650xc2x0 C. to 850xc2x0 C.
Preferably, during the consolidation, the temperature conditions include a stage where the temperature gradient rises at 5xc2x0 C. minxe2x88x921 between 850xc2x0 C to 1375xc2x0 C., and the dopant concentrations are selectively controlled so that thermal deformation is minimised over this temperature range.
Preferably during the consolidation, the temperature conditions include a stage where the temperature gradient falls at 5xc2x0 C. minxe2x88x921 between 1375xc2x0 C. to 650xc2x0 C.
Preferably, during the consolidation stage of at least one layer of said cladding layer, said first core layer and/or said second core layer overlying a doped substrate and/or another doped layer, the temperature s conditions include a stage where the temperature remains above the softening temperature of the underlying substrate and/or other layer in its undoped state for at least 60 minutes.